Advances in Biomimetics 132 500 nm A D B C 10 μm 10 20 30 40 50 60 70 0 2000 4000 6000 8000 10000 (004) (002) 2θ ( o ) Intensity (a.u.) 500 nm A D B C 10 μm 10 20 30 40 50 60 70 0 2000 4000 6000 8000 10000 (004) (002) 2θ ( o ) Intensity (a.u.) Fig. 2. Electrophoretic assembly of gibbsite nanoplatelets. (A) Photograph of a free-standing gibbsite film. (B) Top-view SEM image of the sample in (A). (C) Cross-sectional view of the same sample. (D) XRD patterns of the gibbsite film in (A). Adapted from Lin, Huang et al. 2009. The oriented deposition of gibbsite nanoplatelets in a direct-current (dc) electric field can be understood by considering the charge distribution on the gibbsite surfaces due to different isoelectric points at faces (pH ~ 10) and edges (pH ~ 7). The pH of the bath in the electrophoretic experiments is close to 7, resulting in positively charged surfaces and almost neutral edges. Therefore, the applied electric field exerts a force only on the surfaces of the gibbsite platelets and Brownian motion could provide sufficient torque to re-orient perpendicular particles to face the ITO electrode. Once being close to the electrode, the gibbsite nanoplatelets will be forced to align parallel to the electrode surface as this orientation is more energetically favorable than the perpendicular one. If the duration of the electrophoretic process is long enough, almost all gibbsite platelets can be deposited on the ITO electrode. 4.2 Filling nanoplatelet assemblies with ETPTA After oriented deposition, polymer-gibbsite nanocomposites can then be made by filling the interstitials between the aligned nanoplatelets with photo-curable monomers, followed by photopolymerization. We choose a non-volatile monomer, ethoxylated trimethylolpropane triacrylate (ETPTA, M.W. 428, viscosity 60 cps), to form the nanocomposites. The monomer with 1% photoinitiator (Darocur 1173, Ciba-Geigy) is spin-coated at 4000 rpm for 1 min to infiltrate the electroplated gibbsite film and then polymerized by exposure to ultraviolet radiation. The resulting nanocomposite film becomes highly transparent (Fig. 3A) due to the matching of refractive index between the gibbsite platelets and the polymer matrix. The normal-incidence transmission measurement as shown in Fig. 3B shows the free-standing nanocomposite film exhibits high transmittance (> 80%) for most of the visible wavelengths. As the reflection ( R) from an interface between two materials with refractive index of n 1 and n 2 is governed by Fresnel’s equation(Macleod 2001): Bioinspired Assembly of Inorganic Nanoplatelets for Reinforced Polymer Nanocomposites 133 12 12 R[(n )/( )]nnn = −+ (3) A D B C 10 μm 400 500 600 700 800 0 20 40 60 80 100 Transmission (%) Wavelength (nm) 10 20 30 40 50 60 70 0 2000 4000 6000 8000 (004) (002) Intensity (a.u.) 2θ ( o ) A D B C 10 μm 400 500 600 700 800 0 20 40 60 80 100 Transmission (%) Wavelength (nm) 10 20 30 40 50 60 70 0 2000 4000 6000 8000 (004) (002) Intensity (a.u.) 2θ ( o ) A D B C 10 μm 400 500 600 700 800 0 20 40 60 80 100 Transmission (%) Wavelength (nm) 10 20 30 40 50 60 70 0 2000 4000 6000 8000 (004) (002) Intensity (a.u.) 2θ ( o ) Fig. 3. Free-standing gibbsite-ETPTA nanocomposite. (A) Photograph of a transparent nanocomposite film. (B) Normal-incidence transmission spectrum of the sample in (A). (C) Cross-sectional SEM image of the same film. (D) XRD patterns of the same sample. Adapted from Lin, Huang et al. 2009. we can estimate the normal-incidence reflection from each air-nanocomposite interface to be about 4%. Thus, the optical scattering and absorption caused by the nanocomposite itself is ca. 10%. This suggests the polymer matrix has infiltrated most interstitial spaces between the aligned gibbsite nanoplatelets. The cross-sectional SEM image in Fig. 3C shows the nanocomposite retains the layered structure of the original electroplated gibbsite film. Thin wetting layers of ETPTA (~ 1 μm thick) are observed on the surfaces of the film. The oriented arrangement of the nanoplatelets is also maintained throughout the polymer infiltration process as confirmed by the distinctive (002) and (004) peaks of the XRD spectrum shown in Fig. 3D. 4.3 Composition analysis The ceramic weight fraction of the ETPTA-gibbsite nanocomposite film is determined by thermogravimetric analysis (TGA) as shown in Fig. 4. From the TGA curve and the corresponding weight loss rate, it is apparent that two thermal degradation processes occur. One happens at ~ 250°C and corresponds to the degradation of the polymer matrix; while another occurs at ~ 350°C and is due to the decomposition reaction of gibbsite: 3232 2Al(OH) Al O 3H O→+ (4) Based on the residue mass percentage (45.65%) and assuming the ash is solely Al 2 O 3 , we can estimate the weight fraction of gibbsite nanoplatelets in the original nanocomposite film to be ~ 0.70. Considering the density of gibbsite (~ 2.4 g/cm 3 ) and ETPTA (~ 1.0 g/cm 3 ), the volume fraction of gibbsite nanoplatelets in the nanocomposite is ca. 0.50. The complete Advances in Biomimetics 134 infiltration of ETPTA between the electroplated gibbsite platelets is further confirmed by the selective dissolution of gibbsite in a 2% hydrochloric acid aqueous solution. This results in the formation of a self-standing porous membrane with stacked hexagon-shaped pores, which are negative replica of the assembled gibbsite platelets. Fig. 4. Thermogravimetric analysis of a gibbsite-ETPTA nanocomposite. Adapted from Lin, Huang et al. 2009. 4.4 Mechanical test The mechanical properties of the biomimetic polymer nanocomposites are evaluated by tensile tests. We compare the tensile strength for three types of thin films, including pure ETPTA, gibbsite-ETPTA, and TPM-modified gibbsite-ETPTA. The surface hydroxyl groups of gibbsite nanoplatelets can be easily modified by reacting with 3-(trimethoxysilyl)propyl methacrylate (TPM) through the well-established silane coupling reaction. This results in the formation of surface-modified particles with dangling acrylate bonds that can be crosslinked with the acrylate-based ETPTA matrix. The colloidal stability and the surface charge of the resulting nanoplatelets are not affected by this surface modification process as confirmed by TEM and zeta potential measurement. Fig. 5 shows the tensile stress versus strain curves for the above three types of films. The gibbsite-ETPTA nanocomposite displays ~ 2 times higher strength and ~ 3 times higher modulus when compared with pure ETPTA polymer. Even more remarkable improvement occurs when TPM-gibbsite platelets are crosslinked with the ETPTA matrix. We observe ~ 4 times higher strength and nearly one order of magnitude higher modulus than pure polymer. This agrees with early studies that reveal the crucial role played by the covalent linkage between the ceramic fillers and the organic matrix in determining the mechanical properties of the artificial nacreous composites. We also conduct a simple calculation to evaluate if the measured mechanical properties of the gibbsite-ETPTA nanocomposites are reasonable. For a polymer matrix having a yield shear strength τ y and strong bonding to gibbsite nanoplatelet surface (e.g., TPM-modified gibbsites), the tensile strength of the composite (σ c ) can be calculated using the volume fraction of nanoplatelets (V p ), the nanoplatelet aspect ratio (s), and the tensile strength of the nanoplatelets (σ p ) and of the polymer matrix (σ m ), as(Bonderer, Studart et al. 2008) Bioinspired Assembly of Inorganic Nanoplatelets for Reinforced Polymer Nanocomposites 135 0.00 0.01 0.02 0.03 0 20 40 60 0.00 0.01 0.02 0.03 0 20 40 60 0.00 0.01 0.02 0.03 0 10 20 30 40 50 60 Strain Gibbiste+ETPTA ETPTA Tensile Stress (MPa) TPM-modified Gibbsite+ETPTA 0.00 0.01 0.02 0.03 0 20 40 60 0.00 0.01 0.02 0.03 0 20 40 60 0.00 0.01 0.02 0.03 0 10 20 30 40 50 60 Strain Gibbiste+ETPTA ETPTA Tensile Stress (MPa) TPM-modified Gibbsite+ETPTA Fig. 5. Tensile stress versus strain curves for plain ETPTA film, ETPTA-gibbsite nanocomposite, and TPM-modified ETPTA-gibbsite nanocomposite. Adapted from Lin, Huang et al. 2009. cPP Pm σαV σ (1 V )σ = +− (5) For the gibbsite nanoplatelet which has a relatively small aspect ratio (s ~ 12 to 18), the factor α in equation 3 can be estimated as y P ατs/2σ = (6) From the above TGA analysis, the volume fraction of gibbsite nanoplatelets in the polymer nanocomposite is ~ 0.50. If we take s = 15, equation 3 can then be simplified as myc 0.5σ3.75τσ += (7) For acrylate-based polymer (like ETPTA), the yield shear strength should be close to its tensile strength. Equation 7 can further be simplified as σ c ~ 4.25σ m . This indicates that the strength of the nanocomposite is about fourfold of the strength of the polymer matrix, agreeing with our experimental result. 5. PVA-gibbsite nanocomposites 5.1 Single-step electrophoretic deposition of PVA-gibbsite nanocomposites The electrophoretic deposition of PVA-gibbsite nanocomposites is also carried out using the same parallel sandwich cell as described above. The high-molecular weight PVA (Mw 89,000-98,000) is neutrally charged in the electrophoretic bath and can be adsorbed on the surfaces of gibbsite nanoplatelets as water-soluble binders to cement electrodeposited gibbsite nanoplatelets together and also prevent the deposits from cracking. Fig. 6A shows a photograph of a PVA-gibbsite nanocomposite formed on an ITO cathode. The film can be easily peeled off from the electrode surface by using a sharp razor blade. The resulting self- standing film is flexible and transparent, which is different from gibbsite deposits. Optical transmission measurement at normal-incidence shows the film exhibits 60-80% transmittance for most of the visible wavelengths. Top-view SEM image in Fig. 6B illustrates Advances in Biomimetics 136 the gibbsite nanoplatelets are preferentially oriented with their crystallographic c-axis perpendicular to the electrode surface. It is very rare to find edge-on platelets. The ordered layered structure is clearly evident from the cross-sectional SEM images as shown in Fig. 6C and 6D. Fig. 6. Electrodeposited PVA-gibbsite nanocomposite. (A) Photograph of a composite film on an ITO electrode. (B) Top-view SEM image of the sample in (A). (C) Cross-sectional SEM image of the sample in (A). (D) Magnified cross-sectional image. Adapted from Lin, Huang et al. 2009. 5.2 XRD and TGA analysis of PVA-gibbsite nanocomposites The oriented assembly of high-aspect-ratio gibbsite nanoplatelets is further confirmed by XRD. Fig. 7 displays a XRD spectrum of an electrodeposited PVA-gibbsite nanocomposite on an ITO electrode. The diffraction peaks from (222), (400), (441), and (662) planes of the ITO substrate are clearly appeared. Other than ITO diffraction peaks, we only observe (002) and (004) peaks from gibbsite single crystals. As the crystallographic c-axis of single- crystalline gibbsite is normal to the platelet surfaces, the (002) and (004) reflection are from gibbsite platelets oriented parallel to the electrode surface. This strongly supports the macroscopic alignment of gibbsite nanoplatelets in the electrophoretically deposited nanocomposites. Thermogravimetric analysis is used to determine the weight fraction of the inorganic phase in the electrodeposited nanocomposites. Fig. 8 shows the TGA curve and the corresponding weight loss rate for the PVA-gibbsite nanocomposite film. An apparent thermal degradation process occurs at ~250°C that corresponds to the degradation of the PVA matrix and the decomposition reaction of gibbsite as shown in Equation 4. Based on the residue mass percentage (53.96%) and assuming the ash is solely Al 2 O 3 , we can estimate the weight fraction of gibbsite nanoplatelets in the original nanocomposite film to be 0.825. Bioinspired Assembly of Inorganic Nanoplatelets for Reinforced Polymer Nanocomposites 137 Fig. 7. XRD patterns of an electrodeposited PVA-gibbsite nanocomposite on an ITO electrode. Adapted from Lin, Huang et al. 2009. Fig. 8. Thermogravimetric analysis of PVA-gibbsite nanocomposites. Adapted from Lin, Huang et al. 2009. 6. PEI-gibbsite nanocomposites Polyethyleneimine, which is a weak polyelectrolyte and contains amine groups, is positively charged under the electrophoretic conditions. The gibbsite nanoplatelets with a small amount of PEI are well dispersed in a water-ethanol mixture solution due to the electrostatic repulsion between particles. However, adding a larger amount of PEI leads to the agglomeration of gibbsite nanoplatelets. To allow the electrophoresis at a controlled deposition rate, as well as the formation of ordered layered structure, gibbsite nanoplatelets must be stabilized in suspensions. Therefore the influence of the PEI concentration on the stability of gibbsite is studied by measuring particle size distribution and zeta-potential. Advances in Biomimetics 138 6.1 Stability of PEI-gibbsite dispersions To prepare the testing solution, (6 – n) mL of 2.0 wt% gibbsite solution is mixed with n mL of 0.3 wt% PEI aqueous solution, where n = 0, 1, 2, 3, 4, and 5. The weight ratio (PEI to gibbsite, R) is calculated as (n × 0.3)/[(6 – n) × 2]. Fig. 9 shows the size distribution of gibbsite nanoplatelets at different R values measured by laser diffraction. The average diameter of the as-synthesized gibbsite nanoplatelets (R = 0) is 150 nm (Fig. 9A), which is smaller than that observed from TEM images. The random mismatch of the surface of nanoplatelets to the incident laser beam reduces the effective diffraction area, resulting in a smaller average diameter. Fig. 9B shows that no significant change in the particle size distribution is observed when a small amount of PEI is added (R = 0.03). However, further increasing of PEI concentration, as shown in Fig. 9C and 9D (R = 0.075 and 0.75, respectively), leads to a larger particle diameter resulting from the flocculation of nanoplatelets. The flocculation at high polyelectrolyte concentration can be explained by the increase in ionic strength, which leads to the decrease in the electrical double-layer thickness and the instability of the colloids. Depletion flocculation also plays an important role. At a high polymer concentration, the polymer concentration gradient between the inter-particle gap and the remainder of the solution generates an osmotic pressure difference, forcing solvent flows out of the gap until particles flocculate(Dietrich and Neubrand 2001). Fig. 9. Particle size distribution of nanoplatelet suspensions at different PEI/gibbsite weight ratios. (A) R = 0, (B) R = 0.03, (C) R = 0.075, and (D) R = 0.75. Adapted from Lin, Huang et al. 2009. Bioinspired Assembly of Inorganic Nanoplatelets for Reinforced Polymer Nanocomposites 139 Electrophoretic mobility and zeta-potential of nanoplatelets in PEI-gibbsite suspensions with different R values are shown in Fig. 10. Zeta-potential is obtained by fitting experimental data using Smoluchowski’s model. The increase of the electrophoretic mobility and zeta-potential when a small amount of PEI is added (R from 0 to 0.03) is due to the contribution of highly charged PEI that possesses a zeta-potential of ~+60 mV in water at neutral pH. Further increasing of PEI concentration results in the decreasing of electrophoretic mobility and zeta- potential due to the particle flocculation as shown in Fig. 9. -0.2 0.0 0.2 0.4 0.6 0.8 1.0 2 3 4 5 6 mobility Zeta Potential (mV) Mobility (μm cm V -1 s -1 ) PEI/Gibbsite, R (wt/wt) 0 10 20 30 40 50 60 70 zeta potential Fig. 10. Electrophoretic mobility and corresponding zeta-potential of nanoplatelets at different PEI/gibbsite weight ratio. Adapted from Lin, Huang et al. 2009. 6.2 Single-step electrophoretic deposition of PEI-gibbsite nanocomposites The electrophoretic deposition of PEI-gibbsite nanocomposite is again performed using a parallel-plate cell. The positively charged nanoplatelets are attracted toward the bottom Au cathode by the electrical force. As gibbsite nanoplatelets have positively charged surface and almost neutral edges under the electrophoretic conditions, the electric force tends to re-orient the gibbsite nanoplatelets to face the electrode. The positively charged PEI molecules are also electrophoretically migrated toward the cathode together with gibbsite and simultaneously sandwiched between nanoplatelets, forming PEI-gibbsite nanocomposite. Ethanol is added to promote particle coagulation by squeezing the electrical double-layer thickness of the gibbsite nanoplatelets. The high pH near the cathode also helps to coagulate nanoplatelets, as well as neutralize the protonated PEI macromolecules. Top-view SEM images in Fig. 11A and 11B show that the electrodeposited nanoplatelets are preferentially oriented with their crystallographic c-axis perpendicular to the electrode surface. The hexagonal shape and the size of the platelets can be clearly seen in Fig. 11B. Cross-sectional SEM images showed in Fig. 11C and 11D provide further evidence of the ordered layered structure. 6.3 XRD and TGA analysis of PEI-gibbsite nanocomposites XRD spectrum of the PEI-gibbsite nanocomposite on an Au electrode is shown in Fig. 12. The diffraction peak from the (002) plane of gibbsite single crystals is clearly appeared. Comparing to previous results, which show diffraction peaks from both (002) and (004) Advances in Biomimetics 140 planes of gibbsite crystals, the weaker diffraction peak from (004) plane is overlapped with the strong diffraction peak of Au. The (004) diffraction peak can be clearly seen by simply replacing Au electrode with Pt (not shown here). As the (002) and (004) diffraction are originated from gibbsite platelets oriented parallel to the electrode surface, the oriented assembly of nanoplatelets is further confirmed. Fig. 11. SEM images of PEI-gibbsite nanocomposite. (A) Top-view image, (B) magnified top- view image, (C) cross-sectional image, and (D) magnified cross-sectional image. Adapted from Lin, Huang et al. 2009. Fig. 12. XRD patterns of an electrodeposited PEI-gibbsite nanocomposite on Au electrode. Adapted from Lin, Huang et al. 2009. TGA is carried out to determine the weight fraction of the organic phase in the nanocomposites shown in Fig. 13. An apparent thermal degradation process occurs at ~250 °C that corresponds to the degradation of the polymer matrix and the decomposition reaction of gibbsite. Based on the residual mass percentage (63.7%) and assuming the ash contains only Al 2 O 3 , the weight fraction of PEI in the nanocomposite film is estimated to be ~0.03, which is close to the organic content of natural nacre consisting of less than 5 wt% of soft biological macromolecules. [...]... porphyrin-funcationalized shell in organic media (Jin, 2003c) CH3 P P OH N n N H N PEI5 050 (n =50 50) PEI5 05 (n =50 5) N n OH H n H 4 sPEI6-200 (n=200) sPEI6-100 (n=100) sPEI4-200 (n=200) Fig 2 Linear PEIs with various chain architectures OH NH 6 N NH P P P N n OH H p-sPEI240 (n=240) 164 Advances in Biomimetics A B C Heating ~ 6-7 nm Cooling 80oC 20oC 10 μm 20 nm Fig 3 Fibrils self-assembled from linear PEI in. .. PEI5 050 with 1 wt% concentration in water by heating-cooling cycles (B) Optical microscope image of branched fibrous bundles of crystalline PEI5 050 in water (1 wt%) (C) TEM image of fibrous aggregates of 0. 25 wt% PEI5 050 in water The breakthrough of the linear PEI-directed inorganic materials synthesis came from the discovery of crystalline morphology of linear PEI in water media (Yuan & Jin, 2005a)... families of proteins have been identified in the cell wall of marine diatom C fusiformis, termed frustulins, pleuralins and silaffins (Pohnert, 2002) Frustulins are a group of calcium-binding glycoproteins being assumed to play a role in cell adhesion to surfaces, gliding of pennate diatoms, and protection against desiccation (Kröger et al., 1994; 1996) The pleuralins (HF-extractable Learning from Biosilica:... self-assembly of linear PEIs 3.1 Linear PEIs, crystallization and crystalline morphology PEI can be divided into branched and linear PEIs according to their chain architecture Commercial branched PEIs are obtained by cationic ring-opening polymerization of aziridine (ethylene imine) in water or water-alcohol mixture with a protonic acid as catalyst The branched PEI is a polyamine containing primary, secondary... responsive thin film (Schmidt et al., 2009), mainly based on the long-chain secondary amine property In contrast, the nature and characteristics of crystalline structure of linear PEI have not been exploited for materials application, due to that the selfassembled information programmed in the crystalline structure of linear PEI has not been discovered until our recent findings (Yuan & Jin, 2005a) Our interest... nm after 11 minutes of etching The etching rate in the given reactive ion etching condition was approximately 40 nm/min 2 min 7 min 11 min 13 min 15 min Fig 7 SEM images of nanostructures on a glass surface etched with different etching times: 2, 7, 11, 13, and 15 min A high-magnification image of the fabricated nanostructures is shown in Figure 8 This SEM image was obtained under environmental SEM... Superhydrophobicity Langmuir, 20 05, 21: 55 49 -55 54 [12] Gu G, Dang H, Zhang Z, Wu Z Fabrication and Characterization of Transparent Superhydrophobic Thin Films based on Silica Nanoparticles Appl Phys A, 2006, 83: 131-132 158 Advances in Biomimetics [13] Hosono E, Fujihara S, Honma I, Zhou H S J Superhydrophobic Perpendicular Nanopin Film by the Bottom-up Process Am Chem Soc., 20 05, 127: 13 458 -13 459 [14] Sun T, Wang... Learning from Biosilica: Nanostructured Silicas and Their Coatings on Substrates by Programmable Approaches 163 delivery In contrast, linear PEIs obtained from the hydrolysis of linear polyoxazoline in acidic and basic conditions (Saegusa et al., 19 75; Jin & Motoyoshi, 1999), are highly crystalline owing to its linear structure Different to the branched one, linear PEI is composed of only secondary amine... 5 10 15 20 25 30 35 40 45 2 theta /degree Fig 4 Silica mineralization by templating self-assembling PEI nanofiber (A) Schematic representation of PEI nanofiber serving as biomimetic template for controlled silica deposition (B) TEM image of unit PEI@silica nanofiber prepared by using aggregates of 2 wt% P5 050 as template and TMOS as silica source (C) TEM image of silica nanotube obtained by calcining... contact depth obtained from the nanoindentation tests The observed Er is in the range of 2.20 to 5. 17 GPa The decrease in Er with increasing contact Fig 14 Reduced modulus of pure gibbsite and PEI-gibbsite nanocomposite measured by nanoindentation Adapted from Lin, Huang et al 2009 142 Advances in Biomimetics depth may be related to the indentation size effects The size effects are explained as a result . Advances in Biomimetics 132 50 0 nm A D B C 10 μm 10 20 30 40 50 60 70 0 2000 4000 6000 8000 10000 (004) (002) 2θ ( o ) Intensity (a.u.) 50 0 nm A D B C 10 μm 10 20 30 40 50 60 70 0 2000 4000 6000 8000 10000 (004) (002) . thin films, superhard coatings and nacres. In a nanoindentation test, a diamond Berkovich indenter is forced perpendicularly into the coating surface. The load- displacement profile is obtained. Science 322 (59 07): 151 6- 152 0. Oliver, W. C. and G. M. Pharr (1992). "An improved technique for determining hardness and elastic-modulus using load and displacement sensing indentation